Mild:
* As frightened as a mouse in a maze
* As frightened as a child lost in a crowd
* As frightened as a deer in headlights
* As frightened as a bird in a cage
* As frightened as a puppy separated from its mother
Moderate:
* As frightened as a rabbit caught in a snare
* As frightened as a cat with its tail caught in a door
* As frightened as a dog in a thunderstorm
* As frightened as a fish out of water
Severe:
* As frightened as a victim of a nightmare
* As frightened as a prisoner facing execution
* As frightened as a child caught in a fire
* As frightened as a soldier in a battlefield
* As frightened as a creature facing a predator
Figurative:
* As frightened as a leaf caught in a hurricane
* As frightened as a ship lost at sea
* As frightened as a butterfly caught in a web
Remember, the best simile will depend on the specific context and the level of fear you want to convey.